Various industries are making use of compressors for pumping, for example, refinery or chemical plants, either to the users or from the producers. There are many industrial applications that require the use of Oil Free Screw (OFS) compressors. An OFS compressor, as the name explains, does not have oil in contact with the screws. However, all these industries share a common problem when using positive displacement OFS compressors, i.e., the occurrence of noise and vibration in the compressors and/or the piping associated with the compressors. A positive displacement compressor is a compressor that may provide a constant volume output. As will be discussed next, vibration due to acoustic resonances may damage or destroy the compression equipment and its supporting process piping and thus should be attenuated and/or eliminated if possible.
In large diameter piping, for example, high-frequency energy can produce excessive noise and vibration, and failures of thermowells, instrumentation, and attached small-bore piping. In severe cases, the pipe itself can fracture. The same is true for the compressors attached to the piping. These problems most often manifest themselves in, screw compressors, and silencers. In the following the screw compressors are discussed for simplicity. A screw compressor typically has two rotors, a male and female rotor. The lobe combination of the rotors can vary as the design intent varies (3×5, 4×6, 6×8).
Two high-frequency energy generation mechanisms predominate in most industrial processes: flow induced (vortex shedding) and pulsation at multiples of running speed (blade-pass in centrifugal compressors and pocket-passing or lobe passing frequency in screw compressors). For the screw compressors, the intermeshing of the helical lobes generates pulsation at the pocket-passing frequency, which is equal to the number of lobes on the male rotor multiplied by the compressor running speed. Normally, the maximum pulsation amplitude occurs at the fundamental pocket-passing frequency. The amplitudes of the higher multiples are typically but not always lower than the amplitude of the primary pocket-frequency. Once this energy is generated, amplification may occur from acoustical and/or structural resonances, resulting in high amplitude vibration and noise.
Silencers may be attached to the inlet and/or outlet of the compressors to reduce the dynamic pressures and noise discussed above. An example of an inlet silencer (dampener) and an outlet silencer attached to a compressor is shown in FIG. 1. The silencers shown in FIG. 1 are volume choke volume type. FIG. 1 shows a compressor system 10 that includes a compressor 20, an inlet pulsation dampener 30 and a discharge pulsation dampener 50. A gas flows into the dampener 30 as indicated by arrow A and the compressed gas flows out of the dampener 50 as indicated by arrow B. The compressor 20 includes, among other things, an inlet cavity 22 and an outlet cavity 24. The inlet cavity 22 has a flange 26, which is connected to the inlet dampener 30 while the outlet cavity has a flange 28, which is connected to the discharge dampener 50.
The inlet dampener 30 has a nozzle 32 characterized by a nozzle length NL. Connected to the nozzle 32, is a cavity 34 that includes a choke tube 36. The cavity 34 has an upper portion 37 characterized by a λ or cross wall length 38 and an axial chamber length 40. The choke tube 36 has a length 42. The inlet dampener 30 has a flange 44 that is connected to the flange 26 of the compressor 20.
The discharge dampener 50 includes a nozzle 52 connected to a cavity 54, that includes a choke tube 56. An axial chamber 58 of the cavity 54, which is directly connected to the nozzle 52, has a length 60 and a λ or cross wall length 62. The nozzle 52 has a nozzle length NL and the choke tube 56 has a length 64. A flange 66 is attached to the nozzle 52 for connecting the nozzle 52 to the flange 28 of the compressor 20. Such a dampener that has a volume 58, a choke 56 and another volume (not labeled) is called a volume choke volume dampener.
However, the dampeners and their components (nozzle, axial chamber, choke tubes, etc.,) need to be sized appropriately to ensure acoustic resonances are not generated within the silencer. This will ultimately result in the reduction in vibration and/or noise. Accordingly, it would be desirable to provide devices and methods that avoid the afore-described problems and drawbacks.